Particular immunophenotypic features favour a diagnosis of FL, but correlation with clinical and morphological characteristics as well as genetic or cytogenetic features is needed. Note that the number of abnormal cells may be low, and even in quite advanced disease the abnormal cells may not be detected in blood.
- The abnormal cells are genuinely small (often similar in size to red cells)
- Cytoplasm may be very scanty
- The nuclei typically have a single cleft
- The circulating cells are generally mature and typically are low in number
Generally, the number of abnormal circulating cells is low; in this case they are numerous. Note (1) The very small size of the neoplastic cells (compare with the surrounding erythrocytes) (2) The cleft down the center of many of the nuclei - not a complex shape or an indentation, nuclei appear almost split (3) The cytoplasm is almost absent. This is the typical morphology of circulating follicular lymphoma cells (centrocytes). In bone marrow or lymph node larger centroblasts will be seen, but these do not generally circulate in significant numbers.
AUDIO COMMENTARY: The morphology of follicular lymphoma cells
IMMUNOPHENOTYPIC RECOGNITION OF follicular lymphoma
The cells do usually not express the typical marker patterns of CLL, MCL or HCL and related disorders. The circulating cells mostly lack the activated phenotype seen in most other LPD types. CD10 expression is characteristic of FL, but may be absent (and is occasionally seen in other disorders).